The invention relates to a digital transmission system having a transmitter and a receiver, the transmitter including a coder for subband coding of a digital signal, such as a digital audio signal, having a given sampling rate F.sub.S, and the receiver including a decoder for decoding the subband coded signal, the coder being responsive to the digital signal, for generating a number of M sub-band signals with sampling rate reduction, the coder dividing the digital signal band into successive subbands of band numbers m(0.ltoreq.m.ltoreq.M-1) increasing with frequency, the subbands having approximately equal bandwidths, the decoder being responsive to the M subband signals for constructing a replica of the digital signal, this decoder merging the subbands to the digital signal band, with sampling rate increase, the coder comprising analysis filter means and a first signal processing unit, the analysis filter means comprising M analysis filters, each having one input and two outputs, the 2M outputs of the filters being coupled to 2M outputs of the analysis filter means for supplying 2M output signals with a sampling rate F.sub.S /M, each analysis filter being adapted to apply two different filterings on the signal applied to its input and to supply each of the two different filtered versions of that input signal to a corresponding one of the two outputs, each one of the 2M filter outputs being coupled to a corresponding one of 2M inputs of the first signal processing unit, the first processing unit having M outputs coupled to M outputs of the coder for supplying the M subband signals, the first signal processing unit being adapted to supply output signals on each of M outputs, an output signal being a combination of at least a number of input signals applied to its 2M inputs, the decoder comprising a second signal processing unit and synthesis filter means, the second signal processing unit having M inputs for receiving the M subband signals and having 2M outputs, the synthesis filter means comprising M synthesis filters each having 2 inputs, and one output coupled to the decoder output, the second signal processing unit being adapted to generate an output signal on each of its 2M outputs, an output signal being a combination of at least a number of input signals applied to its M inputs, each pair of outputs of the second signal processing unit being coupled to a pair of two inputs of a corresponding one of the M synthesis filters, each synthesis filter having one output, each synthesis filter being adapted to apply different filterings on the two signals applied to the two inputs and to supply a combination of the two filtered signals to its output, each output can be coupled to the output of the synthesis filter means for supplying the replica of the digital signal having a sampling rate F.sub.S, the coefficients of the analysis or the synthesis filters being derived from the coefficients of a standard filter having a low pass filter characteristic with a bandwidth approximately equal to half the bandwidth of the subbands, .alpha.(x,y) in a M.times.2M coefficient matrix A being the multiplication coefficient in the first processing unit, with which the input signal applied to the y-th of the 2M inputs of the first processing unit is multiplied before it is applied to the x-th of the M outputs of the first processing unit, .beta.(u,v) in a 2M.times.M coefficient matrix B being the multiplication coefficient in the second processing unit, with which the input signal applied to the v-th of the M inputs of the second processing unit is multiplied before it is applied to the u-th of the 2M outputs of the second processing unit, and to an apparatus for recording and/or reproducing a digital signal, and to a transmitter and a receiver for use in the digital transmission system.
The digital transmission described in the foregoing is known from publication No. 27.2 titled "Polyphase quadrature filters--a new subband coding technique", by J. H. Rothweiler in Proceedings of the ICASSP 83, Boston, the pages 1280-1283. The publication describes a method by means of which the coefficients of the analysis filters and the coefficients of the synthesis filters can be derived from the same standard low pass filter H(f). This filter is called a prototype filter in the publication. The analysis filters have a number of filter coefficients that equals the number of filter coefficients of the synthesis filters. European Patent Application No. 90.201.369.7 (U.S. Ser. No. 532,465, filed May 31, 1990) also gives an explanation how to determine the filter coefficients for the analysis filters and the synthesis filters from a standard low pass filter having an odd number of coefficients in the impulse response. Also in this case, the number of filter coefficients in the analysis filters, equals the number of coefficients in the synthesis filters.
By increasing the numbers of filter coefficients on the transmitter side and the receiver side to the same extent, an improvement of the transmission quality of the known transmission system can be obtained. This however leads to the disadvantage of an increased complexity of the transmission system, on the transmitter side as well as the receiver side.